Planar waveguide grating devices have been widely proposed and implemented within wavelength multiplexing/demultiplexing, routing, and optical add-drop applications for dense wavelength-division multiplexing (DWDM) transmission in advanced optical networks. Commonly, this is accomplished with either an arrayed waveguide grating (AWG), or an etched reflecting or transmissive diffraction grating, such as an echelle grating. Such planar devices have gained widespread market acceptance by leveraging semiconductor manufacturing's cost reductions to optical components that traditionally were assembled from multiple discrete elements with very high labor content. When the device performs a demultiplexing function, multiple signal channels of different wavelengths which are transmitted in an optical fiber are launched into an input waveguide of the device, are exposed to a dispersive element which separates them according to their wavelength, and then each signal channel is directed to a predetermined output waveguide of a plurality of output waveguides. A typical spectral response of such a device is shown in FIG. 1. One of the most desired features of such devices is a spectral response having a wide and flat response within a passband of each signal channel. This feature allows system designers several benefits, including higher modulation frequency or data rate of the transmitted network signals whilst minimizing distortions onto the signals from the elements themselves. Further, a spectral response graph having a flat wide portion throughout the passband is indicative of a device with a large tolerance to wavelength drift of an input signal received at the input waveguide and one that is tolerant to passband wavelength drift of the device resulting from, for example, temperature variations, aging or manufacturing offsets. Also, a spectral response as indicated above reduces the effect of polarization dispersion resulted from the planar waveguide geometry. Moreover, a device having such a flat and wide passband is particularly important in DWDM networks where multiple filters are cascaded and the cumulative passband is much narrower than that of a single stage filter.
It is also highly desirable that the transmission coefficient drops sharply at the edges of the passband within the spectral response so that adjacent channels can be closely spaced without causing unacceptable crosstalk. Evidently, a sharper change in transmission coefficient also results in signals within the passband being passed with approximately equivalent attenuation, thereby, rendering the entire passband similar in response.
In a planar waveguide demultiplexing device, the shape of the spectral response is determined by a convolution of the amplitude distribution at the output focal plane, which is an image of the input waveguide mode profile formed by the dispersive grating, with the mode profile of the output waveguide itself. The channel spectral response is approximately Gaussian shaped when single-mode waveguides are used for both input waveguide and output waveguide in a conventional device. The passband is narrow, the passband top is not flat and the transition is slow resulting in relatively high crosstalk on adjacent channels unless very wide spacings are used, thereby limiting the number of channels within the system.
Many improved designs have been proposed to flatten and widen the passband spectral response. However, they all have limitations and drawbacks.
In U.S. Pat. No. 6,381,052, Delisle et al, propose a variety of different devices for providing flat top spectral response. The design of these devices includes a first wavelength dispersive element, an inversion means and second wavelength dispersive element. Unfortunately, using present manufacturing technology, this type of device is not easily produced in a maimer that is commercially viable.